# An Interspecific Assessment of Bergmann’s Rule in Tenebrionid Beetles (Coleoptera, Tenebrionidae) along an Elevation Gradient

## TL;DR

This study examines how body size and heat conservation patterns in tenebrionid beetles change with elevation, revealing differences between species living in different habitats.

## Contribution

The study provides new insights into how elevation affects body size and surface area-to-volume ratios in ectothermic beetles across different ecological groups.

## Key findings

- Surface area-to-volume ratios decline with elevation in all tenebrionid beetles, suggesting heat conservation is important.
- Body size increases with elevation in xylophilous species but decreases in geophilous species up to 1000 m.
- Resource availability and microclimate stability influence body size patterns differently in beetle species.

## Abstract

Warm-blooded animals (like mammals and birds) living in colder climates (higher latitudes or elevations) tend to have larger bodies compared to their relatives in warmer areas. This pattern has been interpreted in terms of heat budget: since the surface area-to-volume ratio decreases with increasing size, larger animals need to produce relatively less heat to maintain stable body temperatures (Bergmann’s rule). This mechanism cannot operate in cold-blooded animals, like most insects, as they do not produce heat. In these animals, a reverse pattern is frequently observed, as an increased surface area-to-volume ratio may allow for more rapid heating and cooling. However, selection for more stable internal temperatures might lead to smaller surface area-to-volume ratios also in cold-blooded animals, leading to a negative correlation between body size and temperature. An analysis conducted on tenebrionids along an elevational gradient in Central Italy revealed that, on average, their surface area-to-volume ratios decline with increasing elevation, thus indicating the importance of heat conservation. However, while in species living under bark or in rotten wood, body size (mass and volume) tends to increase with elevation, in ground-dwelling species, it declines up to about 1000 m and then increases. This suggests that a reduction in resource availability with increasing elevation limits body size in ground-dwelling species up to a certain elevation but not in those living under bark, which benefit from more microclimatically stable conditions and constant resources and need energy for overwintering.

In endotherms, body size tends to increase with elevation and latitude (i.e., with decreasing temperatures) (Bergmann’s rule). These patterns are explained in terms of heat balance since larger animals need to produce less heat relative to their size to maintain stable body temperatures. In ectotherms like most insects, where this mechanism cannot operate, a reverse pattern is frequently observed, as a higher surface area-to-volume ratio in colder climates may allow for more rapid heating and cooling. However, patterns of increasing body size with decreasing temperatures can also be observed in ectotherms if selection for more stable internal temperatures leads to smaller surface area-to-volume ratios. Data on tenebrionids from Latium (Central Italy) were used to model elevational variations in average values of body size (total length, mass and volume) and surface area-to-volume ratio. Analyses were performed by considering the whole fauna and two ecological groups separately: ground-dwelling species (geophilous) and arboreal (xylophilous) species. The surface area-to-volume ratios declined with increasing elevation in all cases, indicating that the need for heat conservation is more important than rapid heating and cooling. However, in xylophilous species (which typically live under bark), body size increased with increasing elevation, and in geophilous species, an opposite pattern was observed up to about 1000 m, followed by an increasing pattern. This suggests that a reduction in resource availability with elevation limits body size in geophilous species up to a certain elevation but not in xylophilopus species, which benefit from more climatically stable conditions and constant resources and need energy for overwintering.